Even in the absence of giant earthquakes, this planet is still emitting a quiet hum. Researchers have measured it at the ocean’s depths.

Scientists have been aware of this quiet hum for decades, and many seismic stations have measured it on the planet’s surface. No one has measured it underwater during periods without earthquakes, though—there’s been far too much background noise. Now, a team of European scientists has made that measurement using ocean-bottom seismometers.

The project to measure the hum began with 57 seismometers dropped over a four million square kilometer area of the Indian Ocean, and recorded from late 2012 to late 2013. They came up with a way to analyze the data that removed potential confounding factors from the tides and the currents, resulting in a clearer recording of the hum. They published their paper published recently in Geophysical Research Letters.

The hum is the result of vibrations traveling with frequencies between 2.9 and 4.5 mHz, like a wave traveling once every few minutes. Music notes have frequencies a million times higher than that. National Geographicreports that there’s some confusion over the origin, whether they come from ocean waves or the atmosphere.

But actually studying the hum can have important scientific uses.There are way more seismometers on land than there are in the ocean. Adding to the total number could help scientists build better models and offer them a more holistic understanding of this hum.

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Maybe one day, the paper writes, the Earth’s hum can even be used to study the Earth’s deep interior.